As is known, many electronic circuits depend for proper configuration on the reception of an initialization or enable signal which will bring the circuit to its design operating condition, when combined with an appropriate voltage supply. The initialization signal may be provided by a purposely arranged circuit.
In general, an initialization circuit produces a signal which follows the rise in the supply voltage up to a predetermined value, then drops to null voltage as that value is exceeded. The threshold value for this change is referred to as the tripping voltage of the initialization circuit, and it matters that the value of this tripping voltage be appropriate for the design of the circuit to be driven to the "on" state.
It is a well-recognized fact, for instance, that analog circuits generally require higher initialization voltage values than digital circuits. Further, the tripping voltage should be at all times a value that will properly set all the circuits affected by the initialization. With integrated circuits of the MOS or CMOS types, that voltage is also tied to the transistor thresholds. It should be further observed that most initialization circuits become automatically operative in those situations where the supply voltage drops to a level below the tripping voltage and no longer ensures proper operation of the circuit networks. The circuit network would also have to be re-initialized when the supply voltage is restored to the steady-state value. However, with some electronic devices, this mode of operation of the initialization circuits may be an untoward one.
Let us consider the instance of circuit networks making up memory registers programmable by a microprocessor unit. With many registers, a requisite is that their configurations upon turning on meet certain specifications, commonly referred to as the default values, which are designed to set a circuit network for operation in its normal mode. Accordingly, the initialization circuit should be capable of activating the memory register in the default state provided for by the specifications.
It happens, however, that upon the supply voltage attaining the steady-state value, the contents of the memory register may undergo changes, and if significant surges contemporaneously occurred in the power supply, the initialization circuit would re-configure the memory register so as to re-establish the default state, as explained above. Thus, what is beneficial to standard circuits to be initialized would instead harm memory registers for which restoration to the default state represents a stable error that is retained until the next write operation by the program unit.
As a practical example, it may suffice to consider that an elemental memory cell implemented in CMOS technology and operated at 5 volts is apt to retain the logic value which has been stored therein, even at very low supply values of less than 1 volt. This feature is retained over relatively long time periods and even on the occurrence of sharp supply voltage drops due to abrupt perturbation and/or having very short duration. Therefore, should the initialization circuit associated with the memory become operative under such conditions, the value contained therein would be lost even if the memory as such would be normally tolerant of the momentary perturbation.